Numerous aerobic processes have been developed over the years for the biological treatment of municipal waste, including both domestic and industrial sewage, for yielding an environmentally acceptable effluent. One of the widely used aerobic processes for such treatment is referred to as the activated sludge process, in which organic matter contained in the municipal wastes is contacted with an oxygen-containing gas in the presence of suspended biologically active organisms under conditions such that the organic material is converted into a form which can be separated from purified water. In these processes, a portion of the insoluble sludge that is formed is recycled to the aerobic zone. Another such process is the trickling filtration method, in which the microorganisms are fixed to a support.
These activated sludge systems and other aerobic processes usually produce a significant net positive production of sludge containing suspended solids, which must then be discarded on a periodic basis. Such biological sludges are difficult to treat, because they have poor dewatering properties and are highly putrescible. Sludge deposition has thus become an important environmental problem.
Numerous processes have been developed for sludge stabilization, one of which has been anaerobic digestion. In anaerobic processes, the organic material present in the sludge is oxidized to by-products such as organic acids, ammonia, and principally methane. Anaerobic digestion, however, has a high cost of operation, and substantial time is required for the digestion process.
Another process for stabilizing activated sludge is referred to as extended aeration, in which the sludge is contacted in an aerobic digestion zone, and the organic material is oxidized over time. Although extended aeration may offer significant advantages over anaerobic digestion, there are problems associated with such processes because of high operating expenses and capital costs.
Among the many variations in processes associated with the aerobic treatment of municipal waste are the following:
U.S. Pat. Nos. 3,547,814 and 3,670,887 disclose the treatment of sewage wherein gross solids are first removed from the sewage by screening and the remaining waste contacted with an oxygen-containing gas and activated sludge. The '814 patent discloses that anaerobic processes have been used to render the sludge non-putrescible and, as noted, require long-term storage. Another suggested technique for treating such sludge involves extended aeration, which increases the degree of auto-oxidation, with a net reduction of such sludge. Unfortunately, the rate of oxidation was generally too low to have a significant effect on net sludge production. Even with extended aeration and an increased degree of auto-oxidation, particularly at the zero net production of sludge level, problems were presented because of large plant size and high operating costs. To reduce size, these patentees thus suggested using an oxygen-rich gas and a high volatile organic material in the sludge. This resulted in a low sludge yield in the overall process.
U.S. Pat. No. 3,356,609 discloses a process for treating municipal waste wherein the initial sewage is clarified, and the effluent is then enriched with a carbon source and contacted with an oxygen-containing gas and activated sludge in a dispersed culture aerobic reactor.
U.S. Pat. No. 4,246,099 discloses a combination of aerobic/anaerobic processes to reduce and stabilize sludge solids in an activated sludge process. In this process, municipal sludge was initially contacted with an oxygen-containing gas under aerobic conditions to partially reduce the biodegradable volatile suspended solids and then anaerobically digested to partially stabilize the sludge. Sludge reduction to less than 40% of the biodegradable volatile suspended solids introduced to the digestion zone was achieved. The concept of thermal aerobic digestion was referred to as autothermal aerobic digestion (ATAD) wherein the digester was operated at elevated temperatures, e.g., from about 45.degree. C.-75.degree. C., or in the thermophilic range.
U.S. Pat. No. 4,026,793 discloses an aerobic digestion process for reducing the solids content in a biodegradable organic sludge by carrying out the digestion in a vessel maintained at a temperature within the range of 38.degree.-46.degree. C.
U.S. Pat. No. 4,652,374 discloses a modified anaerobic fermentation of municipal waste by effecting hydrolysis and acidification of the sewage and then anaerobically digesting the hydrolyzed sewage under conditions for methane generation.
It is also known in a modified extended aeration activated sludge process in combination with autothermal aerobic digestion (ATAD) to use a hydrolytic assist which comprised the treatment of the effluent from the ATAD reactor with acid and subjecting the resulting hydrolyzed effluent to biological digestion in the initial aeration zone, where the sewage was contacted with an oxygen-containing gas and activated sludge. Proceedings, 17th Conference on Municipal Sludge Management, HMCRI, Boston, Mass., 1907, pp. 71-77.
As can be seen from the review of substantial prior art pertaining to aerobic processes, including activated sludge processes, many variations have been proposed in an effort to reduce or minimize sludge production and to stabilize excess sludge produced by aerobic processes. All of these processes in one way or another become quite complex and may exhibit high operating costs or capital costs in order to achieve that objective. In most cases, it is extremely difficult to modify these processes in such a way that there is substantial sludge reduction, based on original organic input, let alone achieving sludge elimination. The latter goal is one often sought but seldom achieved and typically requires intervening physical separation processes such as dewatering and subsequent incineration. Removal of organics from waste streams via respiration and conversion into microbial mass and its subsequent conversion to water and carbon dioxide is seldom achieved.
In my prior U.S. Pat. No. 4,915,840, which is expressly incorporated herein by reference, there is disclosed an improvement for sludge reduction in an aerobic process wherein municipal waste containing organic matter is biologically digested by contact with an oxygen-containing gas in the presence of biologically active organisms. The basic process is shown in FIG. 1 of the '840 patent, which is reproduced as FIG. 1 hereof, the disclosure of which, as set forth in the '840 patent from column 4, line 42 through column 7, line 20, is incorporated herein by reference thereto. In particular, the biological digestion of sludges in an autothermal aerobic digestion unit (ATAD) is a known process. In autothermal aerobic digester zone 34, air, or other oxygen-containing gas, e.g., high purity oxygen, is introduced through line 36 at a rate sufficient for the autothermal thermophilic aerobic digestion of the suspended solids. In this process, a temperature of from about 35.degree. -75.degree. C. is maintained, and the heat generated in the process should be sufficient to maintain temperature without external heating. These autothermal self-heating units contain the metabolic heat generated and require no external heat addition to maintain the autothermal digester at appropriate conditions. The nonconverted product containing organic material of preselected concentration usually from 0.5 to 2% solids, is removed as effluent from autothermal aerobic digester zone 34 via line 35 and all or a portion charged to initial aeration digester zone 6. The recycle plus recycle from secondary clarifier 12 is adjusted to give the desired preselected sludge value. With appropriate decay in autothermal digester zone 34, no net sludge generation is possible. That portion not charged to aerobic zone 6 is removed through line 39 for disposal.
It is specifically noted that in the process of the '840 patent, as is shown in FIG. 1 hereof, sludge reduction is controlled by means of a portion of thickened biologically activated sludge being contacted in hydrolysis vessel 31 (HYD) with acid, e.g., sulfuric acid or base, e.g., alkali metal hydroxide under conditions sufficient to effect hydrolysis of macromolecular components of the organic cells and effect dissolution of inorganic components. Mild acid hydrolysis is achieved in vessel 31 by adding acid and maintaining a pH in the range of from about 0.5 to 2 at a pressure ranging from atmospheric to about 30 psig at temperatures ranging from about 80.degree. to 130.degree. C. for about 2 to 10 hours, typically about 4 to 6 hours. Alkaline hydrolysis can also be effected, and this is achieved by contacting with alkaline materials, e.g., sodium hydroxide, and maintaining a pH of from about 7 to 12 and a temperature of 20.degree. to 50.degree. C. for about 5 to 12 hours. This hydrolytic assist modifies the cell structure of the macromolecular components and renders them essentially soluble and thereby enhances the ability of the biologically active organisms to effect thermophilic decay within the autothermal aeration digester zone 34. By increasing or deceasing the amount of the thickened sludge subjected to hydrolysis, one increases or decreases the rate of decay for the system, and sludge reduction levels can be controlled by controlling the rate of such decay, and thus, the extent of decay. However, since the temperature conditions within the ATAD unit itself can effect some solubilization of these macromolecular components, to that extent, the prior chemical solubilization by hydrolytic assist can be considered to be redundant or inefficient.
Hydrolyzed sludge not charged to autothermal aerobic digester zone 34 may be treated for removal of phosphorous or nitrogen or may be adjusted in pH for optimizing decay in the autothermal aerobic digestion zone. Hydrolyzed sludge is withdrawn from vessel 31 through line 38 and charged to tank 40 wherein pH, for example, is adjusted upwardly to an alkaline level for precipitation of phosphorus compounds which are then removed through line 42. The balance of material in vessel 40 is removed through line 44 and charged to autothermal aerobic digester zone 34.
In accordance with a further improved process of mine, as disclosed in U.S. patent application Ser. No. 07/668,070, filed on Mar. 12, 1991, now U.S. Pat. No. 5,141,646, the disclosure of which is incorporated herein by reference thereto, sludge is charged directly to an ATAD reactor from a mixing vessel to provide immediate digestion. During periodic quiescent periods, a portion of settled biomass is then removed from the ATAD reactor and charged to a hydrolysis unit for treatment with a strong acid or base solution. The settled biomass is permitted to hydrolyze for a period of time, preferably at least about six hours, and is then returned to the mixing chamber upstream of the ATAD reactor. The hydrolysate is mixed with the incoming sludge which is then fed directly to the ATAD reactor. The incoming sludge neutralizes the hydrolyzed stream to bring it to a desired pH 7. The hydrolyzed sludge, which is above room temperature, also helps to heat up the incoming feed sludge. Periodically, purified decant is removed from the ATAD reactor and returned to the plant.
A particularly preferred embodiment of the process is shown in FIG. 5 of the '646 patent, and is reproduced in FIG. 2 hereof. In this process, the sludge or solid waste comprising approximately 8% solids may be fed to the grinder 86 via line 84 and thereafter to the mixer 54 via line 52. The sludge is thereafter passed via line 56 to an autothermal anaerobic digestion (AAD) unit 88 where methane gas is drawn off via line 90. Optionally (via line 92), settled biomass from the AAD unit may be hydrolyzed in the unit 62 and recirculated to the mixing chamber 54. If necessary, excess sludge may be removed via line 93 upstream of the hydrolysis vessel 62.
The AAD unit 88 is an autothermal anaerobic digestion device. It is similar to the ATAD reactor 58, except that it requires higher input solids concentration and it is anaerobic, so that no oxygen (aeration) is supplied. The AAD unit is designed to extract energy from the sludge or trash prior to ultimate stabilization via composting. Water and/or nutrients may be added to the AAD unit, if desired, via line 96. AAD decant from unit 88 is fed to the ATAD reactor 58 via line 94.
A portion of the ATAD biomass is settled and removed as before, and returned to the hydrolysis unit 62 via line 60, the hydrolyzed stream feeding into mixer 54 via line 66. Purified decant from the ATAD reactor may be returned to the plant via line 70, or introduced into a nutrient removal device 72, as described above. Treated decant is returned to the plant via line 78.
The search has therefore continued for improved processes for treating organic wastes and sludge materials so as to reduce the generation of sludge and to do so in a more economical and simplified process.